The present invention relates to a process for the molding of metal free dental restorations such as crowns, bridges, inlays, onlays, etc., from glass-ceramic materials. The invention is also concerned with apparatus which permits dental restorations to be easily molded from glass-ceramic materials and, lastly, the invention relates to the resulting dental restorations.
In the prior art, various methods have been used to form metal free dental restorations from glass-ceramic materials. Glass ceramic materials because of their strength, translucency, non-toxicity and other physical properties are ideal materials for use in forming dental restorations. Because of their suitability, glass-ceramic materials have been used to form dental restorations for at least eighty years. In the prior art, the most widely used means for forming dental restorations from mixtures of glass and ceramic materials is a process which utilized a slurry of glass and ceramic particles. In this process, a die is formed which is an exact replica of the remaining portion of the tooth to which the restoration is to be secured. It is understood by one skilled in the art that the remaining portion of the tooth has been prepared by the dentist in such a manner that the attachment of the restoration is facilitated. To start the process, the dentist takes an impression of the prepared tooth to create a negative impression of the prepared tooth or teeth. This negative impression is then packed with a material to form a positive impression of the prepared tooth or teeth. This positive impression is called a die. Platinum foil is then pressed over the die to form a matrix which is essentially a foundation on which the dental restoration is built. In order to form up the dental restoration, many layers of a slurry of a particulate glass-ceramic material are applied to the platinum foil matrix. As multiple layers of the slurry are built-up and dried, a semi-solid structure is formed which can be carved into shape of the desired dental restoration. Once the desired shape is achieved, the structure is then removed from the die. At this stage, the structure is referred to as a green structure. The structure is then fired. During the firing process, the particulate glass-ceramic material fuses into a solid mass. Because the green structure is formed from multiple layers of the dried glass-ceramic material, uneven fusing may result. As a result of this uneven fusing, the physical properties of the finished restoration may be detrimentally affected, thereby creating an inferior dental restoration. Further, as can be seen from the above description, the overall process is very labor intensive.
Other methods for forming the green restoration have been considered in the prior art, for example, in U.S. Pat. No. 2,196,258, a mixture of particulate glass and ceramic materials which incorporates a binder is packed into a flexible mold to form a green structure which is then fired to form a finished structure. Again, because the process entails the fusing of particulate material, uneven fusing may result and, hence, a weakened and inferior dental restoration may result.
To overcome the problems as described above, the trade has recognized that in order to produce strong, translucent metal free dental restorations, it would be desirable to form these restorations directly from a homogenous, molten glass-ceramic material. It was realized that it may be possible to produce a satisfactory restoration by forcing a molten or plastic glass-ceramic material into a mold having a cavity in the form of the desired dental restoration. The prior art further recognized that the glass-ceramic material could be introduced into the cavity when the glass-ceramic material was in the liquid or in the plastic state.
A constant goal of the prior art as described above was to effect the molding process in a quick and efficient manner, and in a manner that produces a dental restoration that has excellent definition and fit. In dental restorations, definition is extremely important. In order to have a satisfactory restoration, the finest details of the original tooth must be reproduced. For example, for a dental restoration to be successful, the margins must be sharp and well defined. It is in this area that the prior art molding processes are deficient, in that it was not possible to achieve the desired degree of definition.
Further, it is desirable to produce a dental restoration in a short period of time in order to efficiently utilize the overhead of the dental laboratory, and in order to minimize the labor content of the dental restoration.
Dental laboratories are not typically well funded operations. Therefore, in order to keep costs to a minimum, it is highly desirable that a suitable process for forming dental restorations utilize equipment which is relatively inexpensive. While the above described process fits this requirement, the below described DI-COR process does not.
As is discussed above, there are several prior art processes for the manufacture of dental restoration from glass-ceramic materials. A recent addition to the prior art is the DI-COR process as sold by Dentsply International, Inc., of York, Pa. In this process, a dental restoration is formed by centrifugal casting of a molten glass-ceramic material. This process is further described in U.S. Pat. No. 4,431,420, issued Feb. 14, 1984, and related patents. Centrifugal casting has been extensively used in the casting of metals principally by the lost wax process. Further, this process has been imminently successful for hundreds of years for use in conjunction with metals. This success results from the fact that molten metals have very low viscosity and high density in the molten state, hence, they function very well in centrifugal casting processes. That is, because molten metals have a high density and a very low viscosity in the molten state, centrifugal force is adequate for purposes of injecting the molten metal into a preformed mold cavity. In an attempt to produce dental restorations which have high definition, the above-mentioned DI-COR process uses centrifugal force to form the desired dental restorations from a molten glass-ceramic material. Molten glass-ceramic materials have a much higher viscosity, and a much lower density when compared to molten metals. For this reason, it is not possible to consistently drive a molten glass-ceramic material by centrifugal force alone into a mold in order to produce a satisfactory dental restoration. That is, a molten glass-ceramic material cannot be driven by centrifugal force into a mold cavity with sufficient force in order to always get the required definition, which is necessary to form a satisfactory dental restoration. It is well recognized by one skilled in the art, that in order to have a satisfactory dental restoration, excellent definition must be achieved in order to recreate the desired margins, which are needed for the proper fit of a dental restoration into the human mouth.
Further, the DI-COR process is deficient as to the coloration of the glass-ceramic material utilized. The resulting DI-COR dental restoration had an undesirable white color and must be glazed in order to produce satisfactory human coloration. As a result, the coloration is only on the surface of the dental restoration. If adjustment by grinding is needed in the final installation of the restoration into the human mouth, the glazing is removed, thereby exposing the whitish base which contrasts with the glaze. This contrast is very unsatisfactory from an esthetic point of view.
In contrast to this deficiency, the restoration of the subject process is adapted to utilize glass-ceramic materials, wherein the coloration of the resulting dental restoration throughout approximates human tooth coloration. Hence, if grinding is necessary in final fitting, contrast between the surface of the dental restoration and the underlying base is not observed.
Undesirable contrast can also result from normal wear, wherein as a result of the grinding action of one tooth against another, the glaze is worn away. Again, this is not a problem in this invention as the preferred glass-ceramic material has a uniform, natural coloration throughout. It should also be noted that the restoration of the subject invention may be glazed to achieve the exact shade desired.
In contrast to the above-discussed prior art processes, the process of the subject invention utilizes a positive, mechanically applied force for purposes of injecting the molten dental glass-ceramic material into the preformed mold cavity.
For a dental glass-ceramic material to be satisfactory for use in the formation of dental restorations, the material should incorporate many of or all of the following properties:
(1) It must be inert and non-toxic in an oral environment.
(2) It must have sufficient structural integrity to resist the forces of mastication, and, generally, must have a 3-point M.O.R. of at least 30,000 PSI.
(3) It should be capable of being formed into forms which are compatible with the human anatomy using simple equipment.
(4) It should have esthetic qualities (coloration similar to human teeth with a slightly translucent appearance), which are compatible with human teeth, and, hence, should be monolithic or glazable.
(5) The glass-ceramic material must not absorb moisture or stain, and it must be stress corrosion resistant.
(6) The glass-ceramic material should have wear characteristics which are similar to natural human teeth, and should be compatible with other dental materials.
(7) The glass-ceramic must have dimensional stability and resist thermal shock during processing, and, in particular, it must have dimensional stability during subsequent heat treating processes wherein recrystallization is effected.
(8) The glass-ceramic material should be compatible from a thermal expansion point of view with metals, stains, glazes, etc., as are conventionally used-to-form dental restorations.
(9) In order to create an esthetically pleasing dental restoration, it may be necessary to alter the final dental restoration to the exact shape and shade desired. In order to effect these alterations, the dental restoration must be heated to a temperature of at least 750xc2x0 C. for each operation. Therefore, a satisfactory glass-ceramic material must be capable of withstanding multiple heat cycles to at least 750xc2x0 C.
(10) A suitable glass-ceramic material must be capable of retaining its structural integrity during heat treating.
(11) A suitable glass-ceramic material should have:
a. Coefficient thermal expansions (C.T.E.) of 9 to 14.5xc3x9710xe2x88x927/xc2x0 C.;
b. Translucency of 2.5 to 4.0 on a visible scale of 0 (clear) to 5 (opaque) and overall beauty;
c. M.O.R. of at least 30 K.S.I. average;
d. Ability to be heat treated to 750-950xc2x0 C.;
e. Structural integrity during heat treatment;
f. Meltability and formability;
g. Chemical durability in an oral environment.
The subject invention includes glass-ceramic materials which meet the above set forth criteria.
In accordance with the above description, it is obvious that in accordance with the prior art, it is difficult, if not impossible to form top quality dental restorations by molding glass-ceramic materials in an inexpensive and efficient manner.
The process, apparatus, compositions and dental restoration of the subject invention provide improvements over the prior art. By use of the process of the subject invention, glass-ceramic materials can be readily molded into dental restorations. Further, the apparatus of the subject invention is relatively inexpensive and is easy to utilize. The compositions of the subject invention are highly advantageous in that they produce esthetically pleasing dental restorations which are chemically inert in the human mouth, and have outstanding strength properties. Further, the compositions of this invention are advantageous in that when dental restorations formed from these compositions are heat treated, they maintain their structural integrity. Further, these dental restorations are capable of withstanding multiple heat cycles to at least 750xc2x0 C., possess thermal expansions which are compatible with the existing porcelains and therefore, these dental restorations can be readily altered using conventional porcelain materials.
Likewise, dental restorations in accordance with the present invention, have acceptable coloration after heat treating, and can be used at that stage without further cosmetic treatment in the mouth. In order to enhance the esthetic properties of the resulting dental restoration, the dental restoration can be readily altered using porcelain materials to achieve any desired effect.
Accordingly, it is an object of the present invention to provide an efficient process whereby dental restorations may be molded from glass-ceramic materials.
It is a further object of the present invention to provide glass-ceramic dental restorations which have outstanding strength and are esthetically pleasing.
It is also an object of this invention to provide glass-ceramic compositions which are suitable for the formation of dental restorations which have a high degree of micro-structural control during the development of the crystals, thereby permitting great flexibility in forming these dental restorations.
It is still another object of this invention to provide a glass-ceramic material which will maintain its structural integrity during heat treating, and, in particular, will not slump or sag during heat treating outside of the investment.
Another object of the present invention is a glass-ceramic material which is suitable for the formation of dental restorations, and which is capable of maintaining its translucency during multiple firing cycles to approximately 750xc2x0 C.
Still another object of this invention is a process for forming a ceramic dental restoration wherein the process comprises:
(1) Placing a glass-ceramic material in a heat-pressure deformable crucible;
(2) Heating the crucible and glass-ceramic material to a temperature at which said crucible becomes heat-pressure deformable, and the glass-ceramic material is moldable;
(3) Bringing the heated crucible into contact with a mold having a preformed cavity therein;
(4) Continuing to move the crucible into contact with the mold, thereby causing the crucible to deform against the mold and causing the moldable glass-ceramic material to be injected into said cavity, thereby forming a dental restoration;
(5) Cooling the mold and the ceramic dental restoration therein;
(6) Removing the formed ceramic restoration from the mold;
(7) Heat treating the dental restoration; and
(8) Finishing the dental restoration.
Also, and object of this invention is to form a heat deformable crucible having a base and a side portion, wherein said crucible is formed by the heat sintering of a particulate mixture of glass and a metal oxide. A preferred composition comprises from about 27 to 31 percent glass, and from about 69 to about 73 percent of a metal oxide.
Another object of this invention is a dental restoration which is formed from a glass-ceramic material having the following composition (by weight percent):
Still another object of this invention is a dental restoration which is formed from a glass-ceramic material having the following composition (by weight percent):
Lastly, an object of the present invention is to create a Lithium Disilicate glass-ceramic material which has excellent chemical durability, and as such, will not deteriorate when exposed to the fluids in the human mouth.
These, and other objects, features, and advantages of the present invention, will become apparent from the detailed description herein.
The advantages of the present invention can be more clearly understood from the following description taken together with the drawings.